1. Field of the Invention
The field of the invention generally relates to optical devices, and more particularly, to lithium niobate (LiNbO.sub.3) optical waveguide couplers and switches.
2. Related Art
Lithium niobate is the material of choice for many optical switching applications. It has currently reached a state of device and process maturity that exceeds other competing technologies. There are many potential applications in which lithium niobate waveguide devices are in systems to switch, modulate, or process optical signals with one specific state of polarization. (For example, see Watson et al., "A Low Voltage 8.times.8 Ti:LiNbO.sub.3 Switch with a Dilated-Benes Architecture", IEEE Journal of Lightwave Technology, Vol. 8, No. 5, May 1990.) Single polarization modulators, switches, and switch matrices in lithium niobate are now common. However, single polarization causes a problem from a systems applications point of view, in that polarization of the light must be maintained throughout the system, which is both difficult and expensive. System architectures have been designed that split, separately switch, and then recombine the two states of polarization to avoid polarization dispersion problems. (For example, see U.S. Patent to Bergland et al., U.S. Pat. No. 5,317,658). However, such systems significantly increase system cost because they double the number of required switch modules.
To eliminate the problems associated with single polarization switches, polarization-independent modulators and switches have been developed. An example 2.times.2 lithium niobate polarization-independent switch is part number A-4335, sold by AT&T Lightwave Business Unit in Breinigsville, Pa. An example of a large array of polarization-independent switches is Granestrand et al., "Pigtailed Tree-Structured 8.times.8 LiNbO.sub.3 Switch Matrix with 112 Digital Optical Switches", IEEE Photonics Technology Letters, Vol. 6, No. 1, January 1994.
These devices eliminate the polarization dependent switch problem but introduce a significant but more subtle problem of polarization dispersion. Lithium niobate is birefringent, which means that two states of polarization pass through the waveguide at different speeds thus causing them to spread in time. This phenomenon is known as polarization dispersion. It can limit system performance by spreading out high speed data signals. When adjacent pulses start to overlap, data transmission errors occur. Polarization dispersion can also result in non-birefringent materials when the waveguide fabrication process introduces birefringence. This birefringence is generally less than material birefringence but it can have an impact on devices. Therefore, what is desired is a technique that compensates for polarization dispersion and eliminates its deleterious effects.